This invention relates to a method of removing at least partly a metallic coating layer from scrap metal pieces, such as sheet pieces, having a core and the coating layer on the core, for example aluminium brazing sheet scrap. The coating layer has a lower melting temperature than the core. In this specification, TS(coat) and TL(coat) are defined as the solidus temperature and the liquidus temperature respectively of the metallic coating layer. TS(core) and TL(core) are defined as the solidus temperature and the liquidus temperature respectively of the core, i.e. the metal pieces without the metallic coating layer.
At different points in the production cycle during the manufacture of metal sheet parts which are provided on at least one side with a metallic coating, such as for example aluminium brazing sheet, coated scrap metal parts occur, such as rejected semi-finished products, cut-off edge parts, parts from the rolling process, stamping waste, etc. Processing such coated scrap metal parts poses various problems. For instance when melting coated scrap metal parts of brazing sheet, in which the cladding comprises an AlSi alloy, an excessive Si-level occurs in the aluminium melt obtained, so that the aluminium melt is no longer immediately suitable for the manufacture of a core alloy for brazing sheet. The aluminium melt is only suitable for lower grade applications unless the Si content is reduced by diluting the aluminium melt with pure aluminium, which is a costly solution.
EP-A-727499 describes a method in which coated metal pieces, particularly galvanized steel pieces, are agitated in a vessel together with abrasive elements, e.g. Al2O3 bodies, so as to cause multiple collisions. This removes the very thin zinc layer. Liquid, such as water or alkaline solution may also be present in the vessel. In some examples the process temperature is given as 80xc2x0 C. This process is not thought suitable for relatively thick coating layers.
U.S. Pat. No. 4,203,762 discloses a method of processing bimetallic scrap, specifically steel/aluminium sheets, to recover aluminium without forming iron-aluminium intermetallic compounds. The scrap is rapidly heated by radiant heat energy in order to pass quickly through a critical temperature range starting at 450xc2x0 C. to a temperature above the melting point of aluminium to render the coating liquid. The liquid coating is removed by gravitational drainage, for which purpose the scrap may be agitated. This process requires a high-temperature energy source and careful control.
It is an object of the invention to provide a method for effectively removing, at least partly, a metallic coating from scrap metal pieces carrying the metallic coating.
According to the present invention there is provided a method of removing at least partly a metallic coating layer from scrap metal pieces having a core and said coating layer on the core, the coating layer having a lower melting temperature than the core, comprising the step of agitating the scrap metal pieces in a container together with a plurality of abrading elements so as to cause multiple collisions therebetween, whereby said coating layer is at least partly removed. The method is characterised in that during the agitating the container temperature is a temperature T in the range Ts(coat) less than T less than TL(core) wherein Ts(coat) is the solidus temperature of the coating layer and TL(core) is the liquidus temperature of the core. Preferably the container temperature T is in the range TL(coat) less than T less than TS(core) in this way, a single feedstock of the coated scrap metal pieces may be brought into motion in such a way that constantly changing variations in speed occur between the coated scrap metal pieces and the abrading particles. It has been found that treating such scrap in this way removes the metallic coating or cladding surprisingly effectively, by mechanisms such as abrasion. With selection of the specified temperature range, the metallic coating or cladding when exposed to this temperature range appears to be very weak and possibly partly molten and can be removed simply and effectively by the abrasive action.
In the method of the invention, the scrap metal pieces may be introduced into the container at a temperature below the temperature TS(coat), e.g. at ambient or room temperature. Preferably the abrading elements, or at least some of them, are at the container temperature. The scrap metal pieces in this case are heated during the agitation but may not (and preferably do not) fully reach the temperature T. Thus the coating may not become fully molten. It seems that the coating becomes sufficiently weakened by the agitation and the temperature that it is removed from the core, possibly without melting.
It has been found that the method in accordance with the invention is particularly suitable for the removal of at least a part of the metallic coating of aluminium brazing sheet, in which the metallic coating, otherwise known as clad layer or cladding, is an aluminium brazing alloy comprising Si as main alloying element in a range of 5-15 wt. %. Also, layers of aluminium alloys comprising Zn as main alloying element can be removed very effectively.
The details of the method of the invention (e.g. time, temperature, size and type of abrading elements) should be selected so that the desired result of reduction of the amount of coating layer on the cores of the scrap metal parts is achieved. Under certain conditions there is a risk that detached coating particles may re-adhere to the scrap metal parts.
Suitably the abrading elements are lumps or particles of metal, mineral, ceramic or similar hard material. Preferably the abrading elements have irregular shapes such as lumps. Put also regular shapes such as pyramids or prisms can be used. The abrading elements are, for example, selected from Al2O3, SiC, spinel, bauxite, ardenner split, steel slag, and ceramic rotofinish elements with a hardness of at least 7.0 Mohs.
Although abrading elements of other materials may well be suitable, it is preferred to use one of those given above which are inert. The removed coating or cladding parts do not adhere significantly to these elements. Preferably the abrading elements used do not comprise to a significant extent any material that can react with molten aluminium possibly present during the agitation, such as mullite or Fe3Si. When using abrading elements consisting of mullite, Si can be formed and this is disadvantageous for the final Si content of treated and subsequently remolten scrap metal parts.
The largest dimension of the abrasive particles is preferably maximum 100 mm, and more preferably maximum 20 mm. Especially in this range good results are achieved in removing metallic coatings in the given temperature range.
The efficiency of the removal of the clad layer is further improved when using a combination of relatively large abrasive elements (dimensional range 3-20 mm, preferably 4-20 mm) and small abrasive elements (dimensional range  less than 2 mm). The weight ratio of the amount of xe2x80x9clargexe2x80x9d to xe2x80x9csmallxe2x80x9d abrasive elements is in a range of 2:1 to 75:1, preferably 2:1 to 25:1 and more preferably 10:1 to 20:1.
Preferably the scrap metal pieces have maximum dimensions of not more than 200 mm, more preferably in the range 5-200 mm. Each piece may be regarded as having a length, width and thickness, with the maximum dimension being the length, the width being equal to or less than the length, and the thickness being small in comparison with the length and for example several mm, i.e. in the range 1 to 30 mm, preferably 1 to 10 mm. They may be obtained by a mechanical treatment comprising shearing, cutting or chopping.
Bringing the pieces and abrading elements into motion may be by one or both of rotational tumbling and shaking, and is carried out in a simple and effective manner in a rotating drum or vessel with an axis of rotation lying at no more than 45xc2x0 relative to the horizontal. Preferably that the vessel comprises projections directed inwards for bringing the sheet parts into motion during rotation. This embodiment of the method in accordance with the invention can be applied on an industrial scale and employ known apparatus, e.g. apparatus known as rotofinish equipment. Instead of a rotating vessel there may be used vibrational equipment, such as a moving grid.